1. Field of the Invention
The present invention relates to compositions which are suitable for the manufacture of ceramic tiles, e.g. wall tiles.
2. Discussion of the Prior Art
Ceramic wall tiles in the UK are commonly composed of clays, silica (milled to a desired particle size) and limestone which are mixed together with water to produce a slurry, or "slip". The clay slip is then spray dried to produce a homogeneous free flowing granulate. The tiles are formed in a press and then dried and fired in a tunnel kiln with a typical firing cycle of around four days. The fired tiles, known as "bisque", are then ready for the glazing cycle. The glazed product can be fired in a tunnel kiln with cycles from 12 to 20 hours, or in a single layer roller kiln with cycles of 30 to 45 minutes. The resultant tiles comprise a tile body with a fired glaze thereon.
Advances in kiln design and glaze technology have led to the development of single fired tiles. This is where the whole tile bisque and glaze are fired together in one kiln in a single firing cycle. This leads to a cost reduction in the manufacturing process.
Current tile body formulations are compromises between the various manufacturing systems and processes.
White firing tile bodies are composed of (i) ball clays, e.g. from Devon and Dorset, (ii) China clays, e.g. from Cornwall and Devon, (iii) silica in the form of sand, from sedimentary deposits, and (iv) cristobalite, a high-temperature form of silica (SiO.sub.2) which has to be manufactured from silica sand by heating to 1300-1400.degree. C. and (v) limestone.
The silica is needed to ensure the correct thermal expansion of the tile material during firing and the cristobalite to prevent crazing of the tile material. The limestone is used to provide dimensional stability and together with the silica reduces the moisture expansion of the clays, which increases the craze resistance of the tile. White firing clays, in the UK and elsewhere, are subject to moisture expansion, since the fired clay product can absorb water and expand. The typical outcome of this is crazing. The glaze needs to be under compression. Consequently when the product absorbs water and expands the glaze is no longer under compression. The glaze accordingly cracks to form a circular crazing pattern.
Silica is usually added to control moisture expansion, and to increase the thermal expansion of the clay. Free silica at an addition of from 25-35 wt % is typically added to provide a total silica content of from 65-68 wt %.
Silica has a thermal expansion curve such that at a temperature of 573.degree. C. it undergoes a conversion from "type a" quartz to "type b" quartz. This change is accompanied by a size change which in turn imparts high thermal stress on the product, in particular on cooling. Consequently the tile can crack. Cristobalite is added as a replacement for part of the silica. The weight ratio of the silica to cristobalite is typically in the range of from 2:1 to 3.5:1.
Limestone is added to control the size variation of the finished tile, and additionally it imparts control of moisture expansion by controlling the formation of a glass phase in the tile. Limestone is added to give a typical CaO range of from 5-7 wt %.
A typical known tile composition, wherein the components are express as oxides, is:
______________________________________ SiO.sub.2 66-68 wt % Al.sub.2 O.sub.3 13.5-15.5 wt % CaO 5-7 wt % MgO 0.2-0.4 wt % K.sub.2 O 1.0-1.5 wt % Na.sub.2 O 0.15-0.25 wt % Fe.sub.2 O.sub.3 0.6-0.8 wt % TiO.sub.2 0.7-0.8 wt % L.O.I. 5-6 wt % ______________________________________ (L.O.I. means "loss of ignition")
Conventional ceramic tile compositions suffer from a number of disadvantages. The mechanical properties of a fired tile often are not sufficiently good for particular applications. In particular, the thermal shock resistance of the tile body may be insufficiently low. This can cause crazing of the tile when subjected to rapid temperature changes. In addition, the moisture expansion of the tile body can be too high, in turn leading to cracking problems. It is desirable for the tile bodies to be rapidly fired in a single firing cycle but this is not achievable by some tile compositions because of insufficient fluxing action. Moreover, it is required that the variability in the size of the tiles is minimized, but for some known compositions, the tiles can shrink in a variable manner, leading to size variations. Furthermore, the fired tile can have too low a modulus of rupture. Finally, it is desired to avoid the requirement for cristobalite to control the silica expansion. The use of cristobalite to control the silica expansion leads to a high cost element in the tile body formulation due to the inherent high cost of the cristobalite because of its manufacturing process.
The present invention aims at least partially to overcome these problems of the prior art.